Back up Windows itself through Macrium Reflect or Aomei Backupper, using an image file. You'd need to include the System Reserved partition if there is one as it contains the necessary boot files. Paragon Backup and Recovery is another good program. All of these are free.
Avoid incremental images if at all possible. Just do periodic full images, maybe weekly or monthly.
Back up personal data through an ordinary file by file copying program, not using an image. I use FreeFileSync, but there are several free programs that work similarly.
Try to talk your friend into putting his data on a different partition than Windows if he hasn't already done so.